A chemical vapor deposition (CVD) method has been known as a method of forming a porous SiOCH film. However, a coating method is advantageous in terms of simplicity of an apparatus and processes.
In the coating method, first, in order to form the crosslink structure of SiOCH, a precursor is dissolved in a solvent to manufacture a coating solution. Then, the coating solution is coated on a substrate by a spinning method, for example. Then, the substrate is heated so that the solvent evaporates to be removed and the precursor is reacted therewith to form the crosslink structure, for example. Therefore, a silicon oxide film such as the SiOCH film is formed. Then, a plasma process such as etching or ashing is performed on the silicon oxide film to form a recessed portion. Thereafter, a barrier film and a metal wiring are laminated in the recessed portion from bottom to top in that order. Therefore, a circuit is formed. The barrier film is for preventing a metal from being diffused from the metal wiring to the silicon oxide film.
For example, the pores in the silicon oxide film, as illustrated in FIGS. 11A and 11B, are formed by combining a low boiling point compound such as an organic material that evaporates at a low temperature with, e.g., silicon in the precursor (refer to FIG. 11A); by coating the coating solution including the low boiling point compound on the substrate; and by heating the substrate to evaporate the low boiling point compound and to remove the low boiling point compound from the surface of the substrate (refer to FIG. 11B).
Therefore, it is required that the low boiling point compound (i) has a molecule-size same as the designed diameter of pores to be formed in the film, (ii) evaporates at the bake temperature of the coating film, and (iii) has a dispersion property that enables the low boiling point compound to uniformly disperse in the precursor. To that end, huge amounts of time and cost are required for selecting or developing the low boiling point compound.
Since the pores are formed by the low boiling point compound evaporating to leak to the outside of the insulating film, the pores become open pores connected to the outside of the insulating film. Therefore, when the barrier film is formed, the metal may be diffused from the barrier film to the silicon oxide film, thereby deteriorating the insulation property of the silicon oxide film. Further, since plasma enters the inside of the film during etching or ashing, the film is easily damaged. In addition, the mechanical strength (hardness and Young's Modulus) is low.
A technique of cleansing industrial machinery by using bubbles having a micrometer or nanometer size, so called, micro bubbles (nano bubbles) has been disclosed in Japanese patent Application Publication No. 2004-121962 (in particular, paragraph Nos. [0031] to [0037]). Moreover, a technique of stably manufacturing such nano bubbles has been disclosed in Japanese patent Application Publication No. 2005-245817 (in particular, paragraph Nos. [0012] and [0013]). However, detailed means of solving the above problems have not been described in either document.
In addition, the technique of forming a porous low dielectric constant film on a substrate by the plasma CVD method has been disclosed in Japanese patent Application Publication No. 2002-64091 (in particular, paragraph Nos. [0008] to [0010]) and Japanese patent Application Publication No. 2005-500669 (in particular, paragraph Nos. [0025] and [0026]). However, the above problems have not been considered.